Method of drilling a borehole in an earth formation

ABSTRACT

A borehole is drilled in an earth formation using consecutive steps of:
     (a) drilling a first open hole section of a borehole, employing a first drill string extending into the borehole from a surface on the earth, to a casing setting depth;   (b) retrieving the first drill string from the borehole to the surface;   (c) everting a tubular element in the open hole section, wherein axially advancing an inner tube section of the tubular element into the borehole through and in relative axial movement to an outer tube section of the same tubular element;   (d) creating an annular seal between the outer tube section and an inward facing wall of the borehole;   (e) inserting a second drill string through the inner tube section into the borehole; and   (f) further deepening the borehole by drilling a second open hole section of the borehole.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of US Provisional Application No.62/430,075 filed, Dec. 5, 2016.

FIELD OF THE INVENTION

In a first aspect, the present invention relates to a method of drillinga borehole in an earth formation, wherein drilling a first open holesection of the borehole, employing a first drill string extending intothe borehole from a surface on the earth, to a casing setting depth.

BACKGROUND OF THE INVENTION

Traditionally, particularly in the oil and gas industry, casing is setduring drilling of a borehole in the earth. Such casing may aid thedrilling and well completion process in one or more of several ways:

-   preventing contamination of fresh water well zones;-   preventing unstable upper formations from caving in and sticking the    drill string or forming large caverns;-   providing a strong upper foundation to use high-density drilling    fluid to continue drilling deeper;-   isolating different zones, that may have different pressures or    fluids, sometimes referred to as zonal isolation, in the drilled    formations from one another;-   sealing off high pressure zones from the surface, avoiding potential    for a blowout;-   preventing fluid loss into or contamination of production zones; and-   providing a smooth internal bore for installing production    equipment.

In the planning stages of a well, a well engineer may pick strategicdepths at casing will be set in order for drilling to reach the desiredtotal depth. The casing setting depths may for example be based onsubsurface data such as formation pressures, strengths, and makeup, andmay preferably be balanced against the cost objectives and desireddrilling strategy.

With the casing set depths determined, hole sizes and casing sizesfollow. The borehole is drilled in intervals whereby a casing which isto be installed in a lower borehole interval is lowered through apreviously installed casing of an upper borehole interval. As aconsequence of this procedure the casing of the lower interval is ofsmaller diameter than the casing of the upper interval. Thus, thecasings are in a nested arrangement with casing diameters decreasing indownward direction. As a consequence of this nested arrangement, arelatively large borehole diameter is required at the upper part of theborehole. Such a large borehole diameter involves increased costs due toheavy casing handling equipment, large drill bits and increased volumesof drilling fluid and drill cuttings. This is a major drawback oftraditional drilling method using casing as described above.

In some instances, the well design may include liners instead of casing,the difference being that casing typically extends all the way up tosurface, while liner is hung off at the bottom of a preceding casing orother liner. For the purpose of the present disclosure, liner and casingare relevant in the same way and the terms are interchangeable.

SUMMARY OF THE INVENTION

In a first aspect, there is provided a method of drilling a borehole inan earth formation, comprising consecutive steps of:

-   (a) drilling a first open hole section of a borehole, employing a    first drill string extending into the borehole from a surface on the    earth, to a casing setting depth;-   (b) retrieving the first drill string from the borehole to the    surface;-   (c) everting a tubular element in the open hole section, which    tubular element comprises an inner tube section and an outer tube    section connected to each other in a lower bending zone, wherein the    inner tube section runs through the outer tube section and wherein a    wall of the tubular element is, in said lower bending zone at a    lower end of the inner tube section, induced to bend radially    outward and in axially reversed direction so as to form the outer    tube section which thereby is everted compared to the inner tube    section, wherein said everting comprises axially advancing the inner    tube section into the borehole through the outer tube section in    relative axial movement compared to the outer tube section;-   (d) creating an annular seal between the outer tube section and an    inward facing wall of the borehole;-   (e) inserting a second drill string through the inner tube section    into the borehole;-   (f) further deepening the borehole by drilling a second open hole    section of the borehole, employing the second drill string, to a    second depth that is deeper than the casing setting depth.

BRIEF DESCRIPTION OF THE DRAWING

The appended drawing, which is non-limiting, comprises the followingfigures:

FIG. 1 schematically shows drilling of a first open hole section of aborehole;

FIG. 2 schematically shows a quantity of a hardening liquid in the firstopen hole section of FIG. 1;

FIG. 3 schematically shows the first borehole section of FIG. 1 havingan everted tubular element of which a lower bending zone is submergedinto the hardening liquid substance of FIG. 2;

FIG. 4 schematically shows further drilling of the borehole by drillinga second open hole section;

FIG. 5 schematically illustrates a close up of the annular seal and thelower bending zone from FIG. 4;

FIG. 6 schematically illustrates extending of the inner tube section ofthe everted tubular element of FIG. 3; and

FIG. 7 schematically shows coiled tubing used for creating the evertedtubular element of FIG. 3.

The figures are schematic of nature, and not to scale. Like referencenumbers are used for like features.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be further illustrated hereinafter by way of exampleonly, and with reference to the non-limiting drawing. The person skilledin the art will readily understand that, while the invention isillustrated making reference to one or more specific combinations offeatures and measures, many of those features and measures arefunctionally independent from other features and measures such that theycan be equally or similarly applied independently in other embodimentsor combinations.

A method is presently proposed wherein a first open hole section isdrilled to a casing setting depth. The drill string is retrieved, andinstead of setting a casing in the traditional way a tubular element iseverted in the open hole section, wherein an inner tube section of thetubular element is axially advanced into the borehole through and inrelative axial movement to an outer tube section of the same tubularelement. After creating an annular seal between the outer wall sectionand the inward facing wall of the borehole, the inverted tubular elementfunctions as a traditional casing.

A second open hole section can be drilled with a drill string extendingthrough the inner tube section of the tubular element. However, as thetubular element is expanded radially outward when being everted, thesecond borehole section does not have to be smaller than the firstborehole section. Instead, the second borehole section may be cased byfurther everting the same tubular element as before.

Compared to drilling and setting traditional casing, the presentlyproposed method becomes more advantageous for each casing setting depththat is needed to reach the final destination depth.

It is remarked that drilling a mono-diameter well and everting a tubularelement in such well is known and described in numerous publicationsincluding U.S. Pat. No. 7,946,349 and U.S. Pat. No. 9,482,070, andEuropean patent application EP 3034189, the contents of each of which isincorporated herein by reference. However, the methods described inthese publications use a complicated machine wherein the tubular elementis continuously formed on-site from a band of flat metal strip wound ona reel. The flat metal sheet is unwound from the reel, fed to the drillstring and bent around the drill string by means of a bending deviceafter which the adjoining long edges of the bent metal sheet arecontinuously welded together to form a tubular element with alongitudinal welded seam. Accordingly, the tubular element is advancedinto the borehole simultaneously with the drill string.

Such complicated machine is not needed in the present proposal, as thedrill string is retrieved from the borehole prior to everting thetubular element. Thus, the present proposal can enjoy at least some ofthe benefits of the known pipe eversion drilling technology, withouthaving to endure some of its drawbacks. Apart from not needing thecomplicated machine, the proposed approach has many other advantages,including that the tubular element can be seamless and in particular thetubular element can be brought on site as coiled tubing. Moreover, thedrilling operation can be carried out with a standard (vertical)drilling rig and standard drill string.

It is further remarked that fabricating a mono-diameter well usingexpandable tubulars that are expanded by advancing an expansion tool,such as an expansion cone, through the tubulars to expand their diameteris known and described in numerous publications including for exampleU.S. Pat. No. 7,100,685 and U.S. Pat. No. 7,357,188. However, thesemethods typically take longer to complete and require more complex andheavy tools compared to the method presently proposed herein.

It is estimated that expandable tubular technologies may be bettersuited for wells having larger casing sizes and/or where the formationpressures are higher than average. Today, the theoretical limit of thepipe eversion technology is estimated to be somewhere between 7 to 9inch outer diameter (OD) of the (unexpanded) inner tube section. As atypical rule of thumb, for typical tubing/CT the resulting OD of theouter tube section is about one inch larger than the OD of the innertube section, this number being merely an indication as it may vary fromcase to case and tube to tube.

The presently proposed method may thus be competitive for open holeinner diameters of up to about 10 inch, and/or using an inner tubesection having OD of up to 7 to 9 inch, specifically in the range of 4to 9 inch or 4 to 7 inch. It is currently envisaged that the presentproposal may be most competitive against competing technologies forsomewhat smaller sizes of up to about 5.5 or 6 inch OD (for instance, inthe range of 4 to 6 inch or in the range of 4 to 5.5 inch and/or mediumto low pressure wells.

FIGS. 1 to 5 illustrate steps of one way of carrying out the proposedmethod. FIG. 1 schematically shows drilling a first open hole section 10of a borehole 1 in an earth formation 2. A first drill string 5 isemployed, which extends into the borehole 1 from a surface 6 of theearth. Any desired type of drill string may be used, includingtraditional jointed string or coiled tubing (CT). A bottom hole assemblyincludes a drill bit 22, which in this instance comprises a pilot bit 24and an under-reamer 26. Alternatives may be employed, as desired. Thefirst open hole section 10 extends to a casing setting depth D ₁.

Upon reaching the casing setting depth D₁, the first drill string 5 isretrieved to the surface 6. Furthermore, a hardening liquid substance 12may be introduced into the borehole 1. FIG. 2 schematically shows theborehole 1 with the hardening liquid substance 12, after the drillstring 5 has been retrieved. The hardening substance will be employed tocreate an annular seal as will be explained below.

Suitably, the hardening liquid substance is a cement, such as aconcrete-based cement or a neat cement. Nonetheless alternatives existin the market which may be used instead of or in addition to concrete orneat cements, such as resin-based substances (see, for example, “Resinemerging as alternative to cement” an article by Sally Charpiot and PaulJones from OffShore Magazine May 2013 and/or GB2480546A). Resin-basedsubstances for wellbore use are commercially available under the nameWellLock® Resin from Halliburton. Suitable resins may be based onscorch-inhibited crosslinkable polymers using atetrahydrocarbylpiperidin-1-oxyl or alkyloxy (TEMPO) compound or of aderivative, preferably an ether, ester or urethane derivative, of aTEMPO compound. There are also non-cementing substances that can be usedto accomplish the seal in the context of the present disclosure, such asfor instance a clay seal (e.g. bentonite).

Suitably, the borehole 1 also contains a non-hardening wellbore fluid13, commonly used to contain the well. The wellbore fluid 13 maysuitably be a drilling mud. The density of the hardening liquidsubstance may be higher than that of the wellbore fluid 13, so that thehardening liquid substance accumulates in the bottom of the first openhole section 10 around the casing setting depth D₁. The hardening liquidsubstance 12 is conveniently introduced into the borehole by spotting aquantity of the hardening liquid substance through the drill string 5,prior to, or while retrieving the drill string 5 to the surface 6, orduring an interruption of retrieving the drill string 5 when is haspartly been retrieved to the surface 6. Alternatively, the hardeningliquid substance 12 may be cast into the borehole 1 after the firstdrill string 5 has been fully retrieved from the borehole 1. In someinstances, it may be more convenient if the hardening liquid substanceis introduced in the annular space between the outer tube section 9 andthe inward facing wall of the borehole by spotting a quantity of thehardening liquid substance through such annular space from the surface6. This may be accomplished using an appropriate side valve (not shown).The latter option may be an advantageous option in zones that have avery stable hole size that does not interfere in the hardening liquidgetting to the bottom of the hole.

The next step is everting a tubular element 4 in the open hole section.This is illustrated in FIG. 3. The tubular element 4 comprises an innertube section 8 and an outer tube section 9 connected to each other in alower bending zone 14. The inner tube section 8 runs through the outertube section 9. A wall of the tubular element is, in said lower bendingzone 14 at a lower end of the inner tube section 8, induced to bendradially outward and in axially reversed direction, so as to form theouter tube section 9, which thereby is everted compared to the innertube section 8. As seen in cross section, the lower end of the tubularelement 4 has a shape that can be described by two U′s (UU) wherein thewall shows a curve 15. A so-called blind annulus 44 is formed betweenthe inner tube section 8 and the outer tube section 9. The blind annulus44 is an annular space that is closed in the lower bending zone 14 bythe curved wall 15.

An upper end of the outer tube section 9 may be suitably landed on awellhead device 50. This wellhead device 50 may form part of or beintegrated into a blowout preventer (BOP). The outer tubular section 9may be axially fixed to prevent axial movement. For instance, it may beconnected to a ring or flange 59, for instance by welding and/orscrewing, on in the wellhead device 50 or any other suitable structureat surface. Optionally, the outer tube section 10 may be fixed to theborehole wall, for instance by virtue of frictional forces between theouter tube section 9 and the borehole wall as a result of the eversionoperation. Alternatively, or in addition, the outer tube section 9 maybe anchored, for instance to the borehole wall.

Suitably, an upper end of the inner tube section 8 may pass through one,two or more annular seals 56, 58 provided in the wellhead device 50. Theannular seals 56, 58 engage with the outside of the inner tube section 8and allow sliding movement of the inner tube section 8 in its axialdirection, and close off the blind annulus 44. The wellhead device 50suitably comprises a conduit 52 which may be connected to a pump (notshown) for pumping a fluid into or out of the blind annulus 44.

Everting of the tubular element 4 comprises axially advancing the innertube section 8 into the borehole 1 through the outer tube section 9 inrelative axial movement compared to the borehole and the outer tubesection 9. As the inner tube section 8 is advanced downward, the wall 15in the lower bending zone 14 is radially bent over an angle of 180°thereby everting the tubular element 4. The everting operation iscontinued until the lower bending zone 14 is submerged in the hardeningliquid substance 12. By allowing the liquid substance 12 to harden whilethe lower bending zone 14 is submerged into the hardening liquidsubstance 12, an annular seal 7 is created between the outer tubesection 9 and an inward facing wall of the borehole 1. FIG. 3schematically shows the first borehole section of FIG. 1 after havingeverted the tubular element 4 to the point that the lower bending zone14 is submerged into the hardening liquid substance 12. An inside borediameter ID of the inner tube section 8 is also indicated.

It is recognized that there are other technologies available to createthe annular seal. For instance, the inside of the inner tube section 8may locally be provided with a swellable material which swells as itbecomes exposed to a fluid in the borehole. The eversion operation willeventually bring the swellable material to the borehole facing side ofthe tubular element 4 after the lower bending zone 14 has passed throughthe swellable material.

Preferably the hardening liquid substance 12 is retarded, to allow timeto complete the eversion operation before hardening of the liquidsubstance is completed. Various technologies are available to retard ahardening liquid. Retardation over a time span of at least 4 hours,preferably at least 6 hours, more preferably at least 8 hours and mostpreferably at least 12 hours may be selected depending on the situation.

After the annular seal 7 has been created, a second drill string 5′,which may be the same drill string as previously used or another drillstring, may be inserted in the borehole 1 through the inner tube section8 of the tubular element 4. This is schematically shown in FIG. 4.Before inserting the second drill string 5′, the inner tube section 8may have to be cut off circumferentially to remove at least a part ofthe tubular element that is exposed at the earth surface. The part ofthe inner tube section 8 below a cut rim is retained, and access intothe borehole 1 is provided though the cut rim. Such cutting may be donewhile the annular seal 7 is forming (e.g. while the liquid substance 12is hardening).

A second open hole section 20 may then be drilled to deepen the borehole1 to a second depth that is deeper than the casing setting depth D₁. Anyhardened cement that is left in the lower part of the inner tube sectionmay be drilled out and reamed. A drilling annulus 32 is maintainedbetween the inner tube section 8 and the second drill string 5′, whichmay be employed for circulation of a drilling fluid as is common in theart. As the drilling progresses, the drilling annulus 32 extends intothe second open hole section 20.

Suitably, the second drill string 5′ comprises a retractableunder-reamer 26 that has a gauge diameter larger than the inside borediameter ID of the inner tube section. This way the second open holesection 20 can be drilled to a bore diameter that is larger than theinside bore diameter ID of the inner tube section 8. Various types ofretractable under-reamers are available in the market.

During the drilling of the second open hole section 20, the second drillstring 5′ is advanced into the borehole 1 as the borehole 1 is beingdrilled, whereas the tubular element 4 is kept stationary. Specifically,the lower bending zone 14 at a fixed depth, similar to a traditionalcasing which is supported on a casing shoe. If necessary, the inner tubesection 8 may be temporarily secured with slips 35 to the drilling floor40, at least for the duration needed to complete the drilling of thesecond open hole section 20.

The drilling of the second open hole section 20 may be continued until afurther case setting depth D₂ is reached. The procedure may at thatpoint be repeated, as many times as required to reach the finaldestination depth. Accordingly, the currently proposed method involvesintermittently drilling, and further everting of the tubular element 4during the drilling intermissions.

The annular seal 7 isolates any annular space that may exist between theouter tube section 9 and the borehole wall from the drilling annulus 32.By proper selection of the casing setting depth, the proposed method canbe employed to drill in formations with different pressure-gradientregimes. FIG. 5 schematically shows a close up of the annular seal 7 andthe lower bending zone from FIG. 4 after the hardened substance 12 hasbeen drilled out. The borehole should be drilled clean enough such thatany remaining substance 12 will not pose a significant barrier tofurther everting of the tubular element 4.

In order to further evert the tubular element 4, it may be needed toextend the inner tube section 8 with an extension tube 18 as illustratedin FIG. 6. Particularly when employing a seamless tubular element,extending may be necessary as during the drilling of the second openhole section 20 the borehole needed to be accessible for the seconddrill string 5′ through the inner tube section 8. Extension is suitablyaccomplished by sealingly abutting a bottom rim 16 of the extension tube18 to a top rim 17 of the inner tube section 8 as indicated by arrows19. There are various techniques available to sealingly connect welltubulars in abutment, including welding. A schematic welding head 21 isincluded in FIG. 6. The extension tube 18 is preferentially seamless,similar to the tubular element 4 that is already in the borehole 1.

As illustrated in FIG. 7, the tubular element 4 may be provided in theform of a coiled tubing 28. The inner tube section extends to acoiled-up portion 29 of the coiled tubing 28. During eversion, the innertube section is supplied by uncoiling the coiled tubing 28 from thecoiled-up portion 29, while axially advancing the inner tube sectioninto the borehole 1. In order to provide access to the borehole 1 forsubsequent further drilling, the coiled tubing 28 may have to be cut ator near the surface 6. This is preferably done at a suitable locationthat allows the coiled tubing (CT) to be re-abutted after drilling ofthe second borehole section. There is a prejudice in the art that CTdrilling is unsuitable for drilling boreholes in formations with morethan one pressure regime. The presently proposed method breaks with thisprejudice. An important break-through is that the present proposal doesnot require to move different CT sizes to the drilling site. Neitherdoes it require a customized tapering to accommodate estimations ofdifferent length and OD's for the specific borehole design. Today, CT isconveniently available in sizes ranging from 1 inch to about 5.5 inch ODmaximum. Larger sizes exist but become logistically more difficult totransport to site as the coils tend to become larger.

To start the everting operation, it is recommended that a startingsection of the tubular element 4 (whether it is coiled tubing or othertube material) has been pre-everted so that it can be landed on thewellhead device 50 in a pre-everted condition. The pre-eversion tooldoes not have to be on-site.

For purpose of interpretation of the present disclosure it is remarkedthat the term “consecutive” is used only to identify an order of stepsrelative to each other, but it is an open term in any other sense.Accordingly the term should not be construed as excluding thepossibility of having any intermediate steps, or any subsequent steps orpreceding steps. Neither should the term be interpreted as limiting thedefinition of any of the steps.

The terms “first” and “second” as such are not intended as qualifyingterms, but these terms are merely intended to provide a uniquenomenclature every feature in the claim for ease of reference.Accordingly, the “first drill string” and “second drill string” are notimplied to be physically different drill strings; while they may bephysically drill strings the terms may also represent the same physicaldrill string.

The term “surface” is may mean any surface above the borehole. The termthus includes: land surface, ocean floor surface, sea surface and othersuitable surface above the borehole. Suitably, the term “surface” may becharacterized by the location of the wellhead.

The term “casing setting depth” should be interpreted as including areasonable margin as common in the art of well engineering. Casingsetting depth is often determined at a depth where a change in formationpressure gradient occurs. However, the change is usually distributedover a certain depth range. Moreover, an actual casing may be setsomewhat, up to for instance about 10 m (about 33 ft), above the actualbottom of the borehole section, rather than in absolute abutment withthe bottom of the borehole. There may be various reasons for this,including the desire to have the ability to extend wellbore tools in theborehole to below the casing.

Ranges defined herein include the end values of the range.

The person skilled in the art will understand that the present inventioncan be carried out in many various ways without departing from the scopeof the appended claims.

What is claimed is:
 1. A method of drilling a borehole in an earthformation, comprising consecutive steps of: (a) drilling a first openhole section of a borehole, employing a first drill string extendinginto the borehole from a surface on the earth, to a casing settingdepth; (b) retrieving the first drill string from the borehole to thesurface; (c) everting a tubular element in the open hole section, whichtubular element comprises an inner tube section and an outer tubesection connected to each other in a lower bending zone, wherein theinner tube section runs through the outer tube section and wherein awall of the tubular element is, in said lower bending zone at a lowerend of the inner tube section, induced to bend radially outward and inaxially reversed direction so as to form the outer tube section whichthereby is everted compared to the inner tube section, wherein saideverting comprises axially advancing the inner tube section into theborehole through the outer tube section in relative axial movementcompared to the outer tube section; (d) creating an annular seal betweenthe outer tube section and an inward facing wall of the borehole; (e)inserting a second drill string through the inner tube section into theborehole; (f) further deepening the borehole by drilling a second openhole section of the borehole, employing the second drill string, to asecond depth that is deeper than the casing setting depth.
 2. The methodof claim 1, wherein during step (f) the second drill string is advancedinto the borehole as the borehole is being drilled, whereas the tubularelement is kept stationary whereby keeping the lower bending zone at afixed depth.
 3. The method of claim 2, wherein the inner tube istemporarily secured at or near the surface for at least the duration ofstep (f).
 4. The method of claim 1, wherein step (f) is continued untilreaching a further case setting depth; and subsequently carrying outstep (b) again with respect to the second drill string, after whichfurther everting the tubular element into the second open hole section,comprising axially further advancing the inner tube section into theborehole through the outer tube section in relative axial movement withthe outer tube section.
 5. The method of claim 1, wherein the tubularelement is a seamless tubular element.
 6. The method of claim 4, whereinsaid further everting comprises extending the inner tube section with anextension tube by sealingly abutting the extension tube to a top rim ofthe inner tube.
 7. The method of claim 6, wherein said sealinglyabutting comprises welding a bottom rim of said extension tube to thetop rim of the inner tube.
 8. The method of claim 6, wherein the tubularelement and the extension tube are both seamless.
 9. The method of claim4, wherein creating a second annular seal between the outer tube sectionand an inward facing wall of the borehole in the second open holesection.
 10. The method of claim 1, wherein the tubular element isprovided in the form of coiled tubing, and wherein the inner tubesection extends to a coiled-up portion of the coiled tubing, and whereinstep (c) comprises supplying the inner tube by uncoiling the coiledtubing from the coiled-up portion while axially advancing the inner tubesection into the borehole.
 11. The method of claim 1, wherein aftercompleting step (c) and prior to step (e) the inner tube section iscircumferentially cut off whereby removing at least a part of thetubular element that is exposed at the earth surface and retaining apart of the inner tube section below a cut rim.
 12. The method of claim1, wherein after completing step (a) and prior to step (d) a hardeningliquid substance is introduced into the borehole, and wherein step (d)comprises allowing hardening of the liquid substance while at least thelower bending zone is submerged into the hardening liquid substance. 13.The method of claim 12, wherein the hardening liquid substance isretarded to allow time to complete step (c) before hardening of theliquid substance is completed.
 14. The method of claim 1, wherein thesecond drill string comprises an under-reamer having a gauge diameterlarger than an inside bore diameter of the inner tube section to drillthe second open hole section to a bore diameter that is larger than theinside bore diameter of the inner tube section.